Direct current voltage regulation of permanent magnet generator

ABSTRACT

An aircraft power generation unit to generate direct current (DC) power provided to a load includes a permanent magnet generator (PMG) that includes first, second, third and fourth sets of windings, each of the winding sets including three windings and a rectifier section with four six pulse rectifiers the produce outputs of Vdc1 to Vdc4 respectively and a common local output bus. The unit also includes an output bus configured to be connected to the load and including a positive output bus rail and a negative output bus rail and a controller that receives an input signal from at least one of the output sets and selectively couples either the common local output bus and fourth rectifier negative rail to the output bus negative rail and one or more of the first to third six-pulse rectifiers to the output bus positive rail to provide a constant voltage to the load.

BACKGROUND

Exemplary embodiments pertain to the art of power generation and, inparticular, regulating a direct current (DC) voltage generated by apermanent magnet generator.

A conventional DC power generating system utilizes a 3-phasevariable-speed permanent magnet generator (PMG) and an active rectifier.PMG's offers high efficiency and high power density in comparison with awound-field synchronous generator. However, the size of the conventionaltwo-level six switch active rectifier requires large size passivecomponents, such as dc link capacitor and output power quality filter.

To achieve a high power density, multilevel topologies, such as anunidirectional Vienna Rectifier or a bi-directional neutral diodeclamped multilevel converter, have been considered to achieve lowharmonic distortion with moderate switching frequency and reduced EMIemissions compared to a standard 2-level active rectifier/inverter.However, these topologies are complex and expensive.

BRIEF DESCRIPTION

Disclosed is an aircraft power generation unit to generate directcurrent (DC) power provided to a load. The unit includes a permanentmagnet generator (PMG) that includes first, second, third and fourthsets of windings, each of the winding sets including three windings anda rectifier section. The rectifier includes: a first six-pulse rectifierconnected to the first set of windings and having a first rectifierpositive rail and first rectifier negative rail and forming a first DCvoltage (Vdc1) between the first rectifier positive rail and the firstrectifier negative rail from voltage received from the first set ofwindings, a second six-pulse rectifier connected to the second set ofwindings and having a second rectifier positive rail and a secondrectifier negative rail and forming a second DC voltage (Vdc2) betweenthe second rectifier positive rail and the second rectifier negativerail from voltage received from the second set of windings, a thirdsix-pulse rectifier connected to the third set of windings and having athird rectifier positive rail and a third rectifier negative rail andforming a third DC voltage (Vdc3) between the third rectifier positiverail and third rectifier negative rail from voltage received from thethird set of windings; and a fourth six-pulse rectifier connected to thefourth set of windings and having a fourth rectifier positive rail and afourth rectifier negative rail and forming a fourth DC voltage (Vdc4)between the fourth rectifier positive rail and fourth rectifier negativerail from voltage received from the fourth set of windings. The unitincludes a common local output bus, an output bus configured to beconnected to the load and including a positive output bus rail and anegative output bus rail and controller. The controller receives aninput signal from at least one of the output sets and selectivelycouples either the common local output bus or fourth rectifier negativerail to the output bus negative rail and one or more of the first,second and third six-pulse rectifiers to the output bus positive rail toprovide a constant voltage to the load, wherein the controllerselectively couples the common local output bus and fourth rectifiernegative rail to the output bus negative rail based on a speed of thePMG.

In one aspect, according to the unit of any prior embodiment, the speedof the PMG is determined based on a frequency of the input signal.

In one aspect, the unit of any prior embodiment further includes twooutput connection switches and the controller selectively couples byclosing one of the two output switches and opening an other of theoutput switches.

In one aspect, the unit of any prior embodiment further includes: afirst switch coupled between the first rectifier positive rail and thecommon local output bus; a second switch coupled between the secondrectifier positive rail and the common local output bus; a third switchcoupled between the first rectifier positive rail and the common localoutput bus; and a fourth switch coupled between the fourth rectifiernegative rail and the output bus negative rail.

In one aspect, the unit of any prior embodiment further includes: afifth switch coupled between the second rectifier negative rail and thethird rectifier positive rail; a sixth switch coupled between the commonlocal output bus and the negative output bus rail; and a seventh switchcoupled between the first rectifier negative rail and the secondrectifier positive rail. The controller selectively couples the commonlocal output bus to the output bus negative rail by closing the sixthswitch and couples the and fourth rectifier negative rail to the outputbus negative rail by closing the fourth switch.

In one aspect, according to the unit of any prior embodiment, the fourthrectifier positive rail is coupled to the third rectifier negative rail.

In one aspect, according to the unit of any prior embodiment, the sixpulse rectifiers are passive rectifiers.

In one aspect, according to the unit of any prior embodiment, Vdc1 isgreater than Vdc2.

In one aspect, according to the unit of any prior embodiment,Vdc2>Vdc3>Vdc4.

In one aspect, according to the unit of any prior embodiment, Vdc1 isabout double Vdc2.

In one aspect, according to the unit of any prior embodiment,Vdc2=2Vdc3=4Vdc4.

In one embodiment, a method of providing direct current (DC) powerprovided to a load is disclosed. The method includes: generatingalternating current (AC) power with a permanent magnet generator (PMG)that includes first, second third and fourth sets of windings, each ofthe sets of winding including three windings; and converting the ACpower produced by the PMG into a DC output with a rectifier section. Therectifier section includes: a first six-pulse rectifier connected to thefirst set of windings and having a first rectifier positive rail andfirst rectifier negative rail; a second six-pulse rectifier connected tothe second set of windings and having a second rectifier positive railand a second rectifier negative rail; a third six-pulse rectifierconnected to the third set of windings and having a third rectifierpositive rail and a third rectifier negative rail; a fourth six-pulserectifier connected to the fourth set of windings and having a fourthrectifier positive rail and a fourth rectifier negative rail and acommon local output bus. The method also includes: selectively couplingwith a controller either the common local output bus or fourth rectifiernegative rail to the output bus negative rail and one or more of thefirst, second and third six-pulse rectifiers to an output bus positiverail to provide a constant voltage to the load, wherein the controllerselectively couples the common local output bus and fourth rectifiernegative rail to the output bus negative rail based on a speed of thePMG.

In a method of any of prior embodiment, the speed of the PMG isdetermined based on a frequency of the input signal.

In a method of any of prior embodiment, the rectifier section includestwo output connection switches and the controller selectively couples byclosing one of the two output switches and opening an other of theoutput switches.

In a method of any of prior embodiment, the rectifier section furtherincludes: a first switch coupled between the first rectifier positiverail and the common local output bus; a second switch coupled betweenthe second rectifier positive rail and the common local output bus; athird switch coupled between the first rectifier positive rail and thecommon local output bus;

a fourth switch coupled between the fourth rectifier negative rail andthe output bus negative rail; a fifth switch coupled between the secondrectifier negative rail and the third rectifier positive rail; a sixthswitch coupled between the common local output bus and the negativeoutput bus rail; and a seventh switch coupled between the firstrectifier negative rail and the second rectifier positive rail and thecontroller selectively couples the common local output bus to the outputbus negative rail by closing the sixth switch and couples the and fourthrectifier negative rail to the output bus negative rail by closing thefourth switch.

In a method of any of prior embodiment, the fourth rectifier positiverail is coupled to the third rectifier negative rail.

In a method of any of prior embodiment, an output voltage of the firstsix-pulse rectifier is greater than an output voltage of the second sixpulse rectifier.

In a method of any of prior embodiment, the output voltage of the secondsix-pulse rectifier is greater than an output voltage of the third sixpulse rectifier and the output voltage of the third six pulse rectifieris greater than an output of the fourth six pulse rectifier.

In a method of any of prior embodiment, the output voltage of the firstsix-pulse rectifier is about double the output voltage of the second sixpulse rectifier.

In a method of any of prior embodiment, the output voltage of the secondsix-pulse rectifier is about double the output voltage of the third sixpulse rectifier and the output voltage of the third six pulse rectifierabout double the output of the fourth six pulse rectifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 is a schematic of a system that generates and delivers aregulated voltage to a load; and

FIG. 2 is a table showing examples switch configurations to provide aconstant voltage output at different PMG speeds.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the disclosedapparatus and method are presented herein by way of exemplification andnot limitation with reference to the Figures.

Disclosed herein is a power generating system that integrates a 12 phasePMG and a power converter into an affordable high power densityalternator with a good power quality, reduced losses, and low torquepulsation. In contrast to prior systems that employ active rectifiers,the disclosed system in one embodiment utilizes passive rectifiers.

One problem that can occur in PMG systems is that the output voltage isdependent of generator speed and load by employing the teachings hereinwherein four passive rectifier are stacked so that the output voltagesof them can be selectively added together, the output voltage can bechanged kept constant while the PMG changes. This can be done bycontrolling one or more switches that selectively choose which singlerectifier or combination of the four passive rectifiers will contributeto the power provided to the load.

FIG. 1 is schematic of power generation system 100 according to oneembodiment. The system can be part of aircraft or other vehicle and canreceive rotational energy from a prime mover 102 such as a shaftconnected to aircraft turbine, a RAM air turbine or an internalcombustion engine.

The system 100 includes a 12-phase permanent magnet generator (PMG) 104.The PMG 104 includes four sets three-phase windings. In FIG. 1, the PMG104 provides three-phase outputs for each winding that are generallyshown by winding outputs 106, 108, 110 and 112. In this example, thewinding outputs 106 are shown as having three phases, 106 a, 106 b and106 c. The other winding outputs can also be three phase though notspecifically referenced as such in FIG. 1.

Each set of winding outputs 106, 108, 110 and 112 is, respectivelycoupled to a six-pulse rectifier 116, 118, 120 and 122. The rectifiersare passive rectifiers in one embodiment. Each rectifier includes apositive rail and a negative rail. As illustrated, a first rectifier 116includes positive rail 130 and negative rail 132, a second rectifier 118includes positive rail 134 and a negative rail 136, a third rectifier120 includes positive rail 138 and a negative rail 140 and a fourthrectifier 142 includes positive rail 142 and a negative rail 144. Eachof the rectifier is formed in the same manner so on the first rectifier130 is discussed in detail.

The first rectifier 116 includes D1 and D2 that serially connectedbetween the positive and negative rails 130, 132 with output 106 aconnected between them. Similarly, D3 and D4 are serially connectedbetween the positive and negative rails 130, 132 with output 106 bconnected between them and D5 and D6 are serially connected between thepositive and negative rails 130, 132 with output 106 c connected betweenthem.

Based on the three phase input voltages received from the windingoutputs 106 a-106 c, the first rectifier 116 produces a dc outputvoltage that this proportional to the magnitude of the signals onoutputs 106 a-106 c. The voltage is measured between the positive andnegative rails 130, 132 of the first rectifier 116 and is shown andreferred to as Vdc1 herein.

Similarly, based on the three phase input voltages received from thewindings outputs 108, the second rectifier 118 produces a dc outputvoltage that this proportional to the magnitude of the signals onoutputs 108. The voltage is measured between the positive and negativerails 134, 136 of the second rectifier 118 and is shown and referred toas Vdc2 herein. Again, based on the three phase input voltages receivedfrom the windings outputs 110, the third rectifier 120 produces a dcoutput voltage that this proportional to the magnitude of the signals onoutputs 110. The voltage is measured between the positive and negativerails 138, 140 of the third rectifier 120 and is shown and referred toas Vdc3 herein. Lastly, based on the three phase input voltages receivedfrom the windings outputs 112, the fourth rectifier 122 produces a dcoutput voltage that this proportional to the magnitude of the signals onoutputs 112. The voltage is measured between the positive and negativerails 142, 144 of the fourth rectifier 118 and is shown and referred toas Vdc4 herein.

Herein, the PMG 102 is arranged such that each set of winding outputsthat is proportions to another set of windings. For example, the firstset of winding outputs 106 can produce a “full” output, the second setof outputs 108 can produce a “½” output that has an amplitude that is ½of that of the first set of outputs 106. Similarly, the third set ofoutputs 1011 can produce a “¼” output that has an amplitude that is ¼thof that of the first set of outputs 106 and the fourth set of outputs112 can produce a “⅛” output that has an amplitude that is ⅛^(th) of thethat of the first set of outputs 106. In such a case,Vdc1=2Vdc2=4Vdc3=8Vdc4. The particular values can be varied based on therequirements as will be understood by the skilled artisan afterreviewing this document.

The four rectifiers 116, 118, 120 and 122 collectively have an outputVout between nodes 210 and 212. Also provided are a series of switchesthat can be opened/closed to select one or more of the rectifiers 116,118, 120 and 122 to connect to the output Vout. Vout is provided tooutput bus 191 in one embodiment.

To smooth voltage output at Vout (e.g., across output bus 191) an outputcapacitor Cdc can be provided across the output bus. This voltage can bedirectly provided to a load 190 in one embodiment. Optionally, an outputfilter 170 can be provided between Vout and the load 190.

The output filter 170 includes inductances L_(F1) and L_(D1) arrangedserially along a positive rail 192 of the DC output bus 191, andinductances L_(F2) and L_(D2) arranged serially along the return rail194 of the DC output bus 191. Resistances R_(D1) and R_(D2) may furtherbe arranged on the DC output bus, in parallel communication withinductances L_(D1) and L_(D2), respectively. Furthermore, a filtercapacitor C_(F) may be arranged across the DC output bus 191. Also, anEMI filter 198 can also be arranged across the DC output bus 191.

As shown, seven switches SW1-SW7 are provided that can select whichrectifier(s) are coupled to the output bus 191. These switches include afirst switch SW1 coupled between the negative rail 132 of the firstrectifier and common local output bus 220, a second switch SW2 coupledbetween the negative rail 136 of the second rectifier and common localoutput bus 220 and a third switch SW3 coupled between the negative rail140 of the third rectifier and common local output bus 220. A fourthswitch SW4 is coupled between the negative rail 144 of the fourthrectifier 122 and the negative rail 194 of the output bus 191.

Two different possible output connections can be made to the negative orreturn rail 194 of the output bus 191. The first is through switch SW4which couples the negative rail 144 of the fourth reciter 122 to thenegative rail 194 and the other is through a sixth switch SW6 thatcouples the common local output bus 220 to the negative rail 194. Asshown, the positive rail 130 of the first rectifier 116 is coupled topositive rail 192 of the output bus.

Also included are fifth and seventh switches SW5, SW7 that can seriallycouple, respectively, the first and second rectifiers 116, 118 and thesecond and third rectifier 118, 120. The positive rail 142 of the fourthrectifier 122 is connected to the negative rail 140 of the thirdrectifier 120.

Based on the configuration of the switches SW1-SW7 different voltagescan be provided to the output but 191. By altering the switchconfiguration, a constant or relatively constant voltage can be providedto the output bus regardless of generator speed. Several examples areprovided below to make this point more clear.

Consider in a situation where a voltage provided to the load (Vdc) ofover about within 300Vdc within 10% accuracy is desired. The PMG 104could be configured such that Vdc1 is 320V, Vdc2 is 180V, Vdc3 is 80Vand Vdc4 is 40V when the PMG 104 is operating at 20,000 rpm. Such outputlevels can be created by selecting the turns ratio of windings in thePMG 104. Is shall be understood, however that the exact 1, ½, ¼, ⅛voltage ratio can be changed depending on the context.

To provide about 300 V to the load 190, switches SW1 and SW6 be closedand all other switches open. In this manner, Vdc1 and only Vdc1 isprovided o the output but 191. However, consider the case where the rpmof PMG is reduced to 18,000. In such a case, the rectifier output valueswill be lower and as follows: Vdc1 is 288V, Vdc2 is 144V, Vdc3 is 72Vand Vdc4 is 36V. In such a case to ensure over 300V is provided to theload, SW1, SW3 and SW4 can be closed. This will add the voltage acrossthe first and fourth rectifiers 116 and 122 to provide a voltage ofabout 324V across the output bus 191. Similar switch modifications canbe made based on motor speed (in RPM's) as shown in Table 1 below:

TABLE 1 Speed, rpm Vdc1 Vdc2 Vdc3 Vdc4 Sw1 Sw2 Sw3 Sw4 Sw5 Sw6 Sw7 Vdc20000 320 160 80 40 Closed Open Qpen Open Open Ctosed Open 320 18000 288144 72 36 Closed Open Closed Closed Open Open Open 324 16000 256 128 6432 Closed Closed Closed Open Closed Closed Open 320 14000 224 112 56 28Closed Closed Open Closed Closed Open Open 308 12000 192 96 48 24 OpenClosed Closed Closed Open Open Closed 312 10000 160 80 40 20 Open OpenOpen Closed Closed Open Closed 360

The system also includes a controller 240. The controller 240 receivesinputs Vac_a, Vac_b, and Vac_c from one of the sets of outputs. Asshown, these signals are received from the fourth winding outputs 112but any could suffice and only 1 phase may need to be example ratherthan the three. Based on the frequency of these inputs, rpm of the PMGcan be determined. With reference to table 1, the controller can selectthe switch positions to achieve the desired output (e.g., greater thanand within 10% of 300V).

For clarity, Table 1 is also reproduced in FIG. 2. In FIG. 2, when avoltage (e.g, Vdc2, etc) is being applied to the output bus 191, thatvoltage is expressed in bold. For completeness, to add the first andsecond rectifier voltages Vdc1 and Vdc2, switches SW2, SW6 and SW7 areclosed. To add the first, second, and third rectifiers 116, 118 and 120,switches SW3, SW5, SW7 and SW6 are closed.

With reference again to FIG. 1, in another embodiment, a voltage sensor196 that measures Vout and provide the measurement as a Vdc_fdbk signalto the controller. In one embodiment

The term “about” is intended to include the degree of error associatedwith measurement of the particular quantity based upon the equipmentavailable at the time of filing the application.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentdisclosure. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,element components, and/or groups thereof.

While the present disclosure has been described with reference to anexemplary embodiment or embodiments, it will be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope ofthe present disclosure. In addition, many modifications may be made toadapt a particular situation or material to the teachings of the presentdisclosure without departing from the essential scope thereof.Therefore, it is intended that the present disclosure not be limited tothe particular embodiment disclosed as the best mode contemplated forcarrying out this present disclosure, but that the present disclosurewill include all embodiments falling within the scope of the claims.

What is claimed is:
 1. An aircraft power generation unit to generatedirect current (DC) power provided to a load, the unit comprising:permanent magnet generator (PMG) that includes first, second, third andfourth sets of windings, each of the winding sets including threewindings; a rectifier section that includes: a first six-pulse rectifierconnected to the first set of windings and having a first rectifierpositive rail and a first rectifier negative rail and forming a first DCvoltage (Vdc1) between the first rectifier positive rail and the firstrectifier negative rail from voltage received from the first set ofwindings; a second six-pulse rectifier connected to the second set ofwindings and having a second rectifier positive rail and a secondrectifier negative rail and forming a second DC voltage (Vdc2) betweenthe second rectifier positive rail and the second rectifier negativerail from voltage received from the second set of windings; a thirdsix-pulse rectifier connected to the third set of windings and having athird rectifier positive rail and a third rectifier negative rail andforming a third DC voltage (Vdc3) between the third rectifier positiverail and third rectifier negative rail from voltage received from thethird set of windings; a fourth six-pulse rectifier connected to thefourth set of windings and having a fourth rectifier positive rail and afourth rectifier negative rail and forming a fourth DC voltage (Vdc4)between the fourth rectifier positive rail and fourth rectifier negativerail from voltage received from the fourth set of windings; and a commonlocal output bus; an output bus configured to be connected to the loadand including a positive output bus rail and a negative output bus rail;and a controller that receives an input signal from at least one of theoutput sets and selectively couples either the common local output busor fourth rectifier negative rail to the output bus negative rail andone or more of the first, second and third six-pulse rectifiers to theoutput bus positive rail to provide a constant voltage to the load,wherein the controller selectively couples the common local output busand fourth rectifier negative rail to the output bus negative rail basedon a speed of the PMG.
 2. The unit of claim 1, wherein the speed of thePMG is determined based on a frequency of the input signal.
 3. The unitof claim 1, further comprising: two output connection switches, whereinthe controller selectively couples by closing one of the two outputswitches and opening an other of the output switches.
 4. The unit ofclaim 1, further comprising: a first switch coupled between the firstrectifier positive rail and the common local output bus; a second switchcoupled between the second rectifier positive rail and the common localoutput bus; a third switch coupled between the first rectifier positiverail and the common local output bus; and a fourth switch coupledbetween the fourth rectifier negative rail and the output bus negativerail.
 5. The unit of claim 4, further comprising: a fifth switch coupledbetween the second rectifier negative rail and the third rectifierpositive rail; a sixth switch coupled between the common local outputbus and the negative output bus rail; and a seventh switch coupledbetween the first rectifier negative rail and the second rectifierpositive rail; wherein the controller selectively couples the commonlocal output bus to the output bus negative rail by closing the sixthswitch and couples the fourth rectifier negative rail to the output busnegative rail by closing the fourth switch.
 6. The unit of claim 1,wherein the six pulse rectifiers are passive rectifiers.
 7. The unit ofclaim 1, wherein Vdc1 is greater than Vdc2.
 8. The unit of claim 7,wherein Vdc2>Vdc3>Vdc4.
 9. The unit of claim 8, wherein Vdc1 is aboutdouble Vdc2.
 10. The unit of claim 9, wherein Vdc2=2Vdc3=4Vdc4.
 11. Amethod of providing direct current (DC) power provided to a load, themethod comprising: generating alternating current (AC) power with apermanent magnet generator (PMG) that includes first, second third andfourth sets of windings, each of the sets of winding including threewindings; converting the AC power produced by the PMG into a DC output,with a rectifier section that includes: a first six-pulse rectifierconnected to the first set of windings and having a first rectifierpositive rail and first rectifier negative rail; a second six-pulserectifier connected to the second set of windings and having a secondrectifier positive rail and a second rectifier negative rail; a thirdsix-pulse rectifier connected to the third set of windings and having athird rectifier positive rail and a third rectifier negative rail; afourth six-pulse rectifier connected to the fourth set of windings andhaving a fourth rectifier positive rail and a fourth rectifier negativerail; and a common local output bus; providing an output bus configuredto be connected to the load and including a positive output bus rail anda negative output bus rail; and selectively coupling with a controllereither the common local output bus or fourth rectifier negative rail tothe output bus negative rail and one or more of the first, second andthird six-pulse rectifiers to an output bus positive rail to provide aconstant voltage to the load, wherein the controller selectively couplesthe common local output bus and fourth rectifier negative rail to theoutput bus negative rail based on a speed of the PMG.
 12. The methodclaim 11, wherein the speed of the PMG is determined based on afrequency of the input signal.
 13. The method of claim 11, wherein therectifier section includes two output connection switches; wherein thecontroller selectively couples by closing one of the two output switchesand opening an other of the output switches.
 14. The method of claim 11,wherein the rectifier section further includes: a first switch coupledbetween the first rectifier positive rail and the common local outputbus; a second switch coupled between the second rectifier positive railand the common local output bus; a third switch coupled between thefirst rectifier positive rail and the common local output bus; a fourthswitch coupled between the fourth rectifier negative rail and the outputbus negative rail; a fifth switch coupled between the second rectifiernegative rail and the third rectifier positive rail; a sixth switchcoupled between the common local output bus and the negative output busrail; and a seventh switch coupled between the first rectifier negativerail and the second rectifier positive rail; wherein the controllerselectively couples the common local output bus to the output busnegative rail by closing the sixth switch and couples the fourthrectifier negative rail to the output bus negative rail by closing thefourth switch.
 15. The method of claim 11, wherein an output voltage ofthe first six-pulse rectifier is greater than an output voltage of thesecond six pulse rectifier.
 16. The method claim 15, wherein the outputvoltage of the second six-pulse rectifier is greater than an outputvoltage of the third six pulse rectifier and the output voltage of thethird six pulse rectifier is greater than an output voltage of thefourth six pulse rectifier.
 17. The method of claim 16, wherein theoutput voltage of the first six-pulse rectifier is about double theoutput voltage of the second six pulse rectifier.
 18. The method ofclaim 17, wherein the output voltage of the second six-pulse rectifieris about double the output voltage of the third six pulse rectifier andthe output voltage of the third six pulse rectifier about double theoutput voltage of the fourth six pulse rectifier.